1. Field of the Invention
The present invention relates to a method for producing high purity silica and ammonium fluoride. More particularly the invention relates to a method of recovering high purity silica and ammonium fluoride from an impure fluoride-containing source.
2. Description of Related Art
Chemically-combined fluorine typically is present in substantially all phosphorus-containing rock (phosphate rock), such as fluorapatite and mineral phosphates. Generally, such phosphate rock contains as much as 4 wt. percent fluorine. When phosphate rock is reacted with an acid, such as sulfuric acid or hydrochloric acid, much of the fluorine value of the rock is liberated as an undesired by-product in the gaseous phase, e.g., as silicon tetrafluoride. Gaseous silicon tetrafluride also is liberated when phosphoric acid is concentrated, as in the production of phosphate-containing fertilizers or wet process phosphoric acid.
The noxious nature of silicon tetrafluoride requires that it be removed from the gaseous phase to avoid atmospheric pollution. Gaseous silicon tetrafluoride usually is recovered by absorption in water; the gas is passed through water absorption vessels or Venturi scrubbers. Absorption of silicon tetrafluoride in water yields aqueous fluosilicic acid solution and silica precipitate.
In the wet process method of making phosphoric acid, weak phosphoric acid typically is returned to the attack tank. However, in one method known in the art, the weak phosphoric acid is treated with sulfuric acid. The heat of dilution of the sulfuric acid is used to strip, as vapor, fluorine values from the dehydration of fluosilicic acid in the weak phosphoric acid. The fluorine is recovered primarily as silicon tetrafluoride; some hydrogen fluoride is also recovered. This vapor is absorbed in water, yielding fluosilic acid and silica precipitate.
The market value of fluosilicic acid, and of fluosilicates derived therefrom, is not sufficiently high, however, to make their production economically attractive. It has been an object of the prior art to utilize by-product silicon tetrafluoride to produce other products having greater market value. Therefore, attempts have been made to develop commercically attractive uses for this by-product.
U.S. Pat. No. 3,271,107 discloses a process for producing silica pigments from fluosilicic acid, generated by absorbing silicon tetrafluoride in water, by reacting fluosilicic acid with ammonium hydroxide in two stages. In the first stage, a less-than-stoichiometric quantity of ammonium hydroxide is added with high agitation to produce a slurry having a pH of between 6.0 and 8.0 and containing minute silica particles. The unreacted fluosilicic acid in this slurry then is reacted with sufficient additional ammonium hydroxide to provide a final pH between about 8.3 and 9.0. Pigment quality silica precipitate then is separated from the slurry.
U.S. Pat. No. 3,021,194 discloses a process for producing ammonium bifluoride from fluosilicic acid and ammonium fluoride purportedly without undue loss of ammonia or fluorine. Concentrated fluosilicic acid is reacted with ammonium fluoride, or a mixture of ammonium fluoride and sodium or potassium fluoride, to produce aqueous ammonium acid fluoride (ammonium bifluoride) solution and solid alkali fluosilicate, including ammonium fluosilicate. After separating the solution from the solid alkali fluosilicates, solid ammonium bifluoride is recovered by evaporatively concentrating the solution. Alkali metal fluosilicates can be recovered and sold, or can be converted to alkali fluorides by reaction with additional ammonia. Ammonium fluoride is produced and hydrated silica is precipitated by this ammoniation. The silica is indicated for use as a filler, a flatting agent, or as an insecticide provided it contains some sodium fluoride.
Certain uses of silica require very high purity material. For example, silica used in the encapsulation or packaging of electronic computer chips must have extremely low levels of metal impurities. Typical of these uses is in very large scale integrated (VLSI) microchip applications, where chip manufacturers require silica having extremely low concentrations of certain radioactive elements. For example, uranium and thorium concentrations must be on the order of less than 1 part per billion (ppb). The maximum acceptable level of ionic impurities, including cations such as aluminum, boron, calcium, cobalt, chromium, copper, iron, potassium, magnesium, manganese, sodium, nickel, vanadium, and zinc, and anions containing phosphorus and sulfur, also is less than 10 parts per million (ppm), and often is below 1 part per million.
Other uses for high purity silica material include precision laser optics, fiber optics, and advanced ceramics, including diffusion tubes and crucibles. Presently, these requirements are satisfied predominantly by natural silica sources such as quartz. Unfortunately, prior art processes for recovering silica from contaminated fluosilcic acid starting materials have not been satisfactory for producing a product satisfying these stringent purity requirements.
U.S. Pat. No. 4,465,657, for example, discloses a process for producing a purified silica from impure fluosilic acid which basically uses the procedure of the earlier U.S. Pat. No. 3,271,107. Fluosilicic acid is reacted in a first step with a less-than-stoichiometric quantity of ammonium hydroxide to convert some of the acid of ammonium fluoride and silica. The silica precipitate thus produced removes metal ion impurities, presumably at least in part by absorption, from the residual fluosilicic acid solution. The silica precipitate then is separated, and the remaining solution having a lower level of impurities is reacted in a second stage with additional ammonium hydroxide to produce a purified silica precipitate. Optionally, the residual fluosilicic acid solution from the first precipitation stage may be treated with an ion exchange or chelating agent to purify the solution further prior to the formation of the silica precipitate in the second precipitation stage.
A particular drawback of this procedure is that from 40 to 75 percent of the available silica in the fluosilicic acid is used as the vehicle for removing impurities. Thus, only, 25 to 60 percent of the silica values of the fluosilicic acid actually can be recovered in a purified form. Moreover, there is a tacit admission that the two step process does not produce a satisfactory product since it is preferred to treat the solution from the first step process with an ion exchange or chelating agent prior to the second precipitation step.
European Patent Application No. 0,113,137 attempts to avoid the loss in yield of U.S. Pat. No. 4,456,657 by adding a chelating agent directly to the impure fluosilicate acid solution to impove the purity of the silica by sequestering or chelating multivalent metal ions in the solution before ammoniation. Ion exchange also has been used for the same purpose. However, these techniques tend to introduce other impurities, such as alkali metal ions, into the precipitated silica. Additionally, these prior art purification processes rely upon cationic exchangers and metal chelating agents and thus cannot satisfactorily remove the phosphorus and sulfur impurities generally present as anionic species (SO.sub.4.sup.-2 and PO.sub.4.sup.-3) in the fluosilicic acid by-product solutions typically recovered from the acidulation of phosphate rock. Nor can anionic exchange agents be used because the anionic exchange agents significantly decrease the recovery of silica.
High-purity ammonium fluoride is useful as a precursor for making an oxide etchant for electronic applications. Simple evaporation of an aqueous solution of ammonium fluoride liberates ammonia and forms ammonium bifluoride, NH.sub.4 FHF, or NH.sub.4 HF.sub.2. Alternatively, ammonium fluoride is also useful as an ammonium source for diammonium phosphate.
Accordingly, it is an object of the present invention to provide a method for recovering high purity silica and ammonium fluoride from the by-products obtained by the acidulation of phosphate rock.
It is another object of the invention to provide a method for producing high purity silica having metal impurity content below 10 ppm, and preferably below 1 ppm, and having radioactive element concentrations below 1 ppb.
It is also an object of this invention to provide a method for producing high-purity ammonium fluoride solution.